Display device, method of laying out light emitting elements, and electronic device

ABSTRACT

Disclosed herein is a display device in which light emitting elements of a plurality of colors including a light emitting element emitting blue light are formed in each pixel on a substrate on which a transistor is formed for each sub-pixel, and a plurality of pixels formed with sub-pixels of the plurality of colors as a unit are arranged in a form of a matrix, wherein relative positional relation between transistors of sub-pixels of respective light emission colors including blue light and a light emitting section of a light emitting element emitting the blue light is laid out such that distances between the transistors of the sub-pixels of the respective light emission colors including the blue light and the light emitting section of the light emitting element emitting the blue light are equal to each other for the respective colors.

This is a Continuation application of U.S. patent application Ser. No.13/763,745, filed Feb. 11, 2013, which is a Continuation application ofU.S. patent application Ser. No. 12/385,648 filed Apr. 15, 2009, whichin turn claims priority from Japanese Application No. 2008-132854 filedin the Japan Patent Office on May 21, 2008, the entire contents of whichbeing incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Recently, flat-panel type display devices in which pixels (pixelcircuits) are arranged in the form of a matrix have spread rapidly in afield of display devices displaying images. As one of the flat-paneltype display devices, there is a display device using a current-driventype electrooptic element whose light emission luminance changesaccording to the value of a current flowing through the device as lightemitting elements of pixels. As a current-driven type electroopticelement, an organic EL (Electro Luminescence) element utilizing aphenomenon of light being emitted when an electric field is applied toan organic thin film is known.

2. Description of the Related Art

An organic EL display device using the organic EL element aselectrooptic elements of pixels has the following features. The organicEL element can be driven by an application voltage of 10 V or lower, andthus consumes low power. Because the organic EL element is aself-luminous element, as compared with a liquid crystal display devicethat displays an image by controlling the intensity of light from alight source in a liquid crystal in each pixel, the organic EL displaydevice provides high image visibility, and is easily reduced in weightand thickness because an illuminating member such as a backlight or thelike is not required. Further, because the organic EL element has a veryhigh response speed of a few μsec or so, no afterimage occurs at a timeof displaying a moving image.

As with the liquid crystal display device, the organic EL display devicecan adopt a simple (passive) matrix system and an active matrix systemas a driving system of the organic EL display device. However, whilehaving a simple structure, a simple matrix type display device presentsfor example a problem of difficulty in realizing a large andhigh-definition display device because the emission period of anelectrooptic element is reduced by an increase in the number of scanninglines (that is, the number of pixels).

Therefore an active matrix type display device that controls currentflowing through an electrooptic element by an active element, forexample an insulated gate field effect transistor provided within a samepixel circuit as the electrooptic element has recently been activelydeveloped. A TFT (Thin Film Transistor) is typically used as theinsulated gate field effect transistor. The active matrix type displaydevice makes it easy to realize a large and high-definition displaydevice because the electrooptic element continues emitting light overthe period of one frame.

It is generally known that the I-V characteristic (current-voltagecharacteristic) of the organic EL element is degraded with the passageof time (so-called secular degradation). In a pixel circuit using anN-channel type TFT in particular as a transistor that current-drives anorganic EL element (which transistor will hereinafter be described as a“driving transistor”), when the I-V characteristic of the organic ELelement is degraded with the passage of time, the gate-to-source voltageVgs of the driving transistor changes. As a result, the light emissionluminance of the organic EL element changes. This is caused by theconnection of the organic EL element to the source electrode side of thedriving transistor.

This will be described more specifically. The source potential of thedriving transistor is determined by an operating point of the drivingtransistor and the organic EL element. When the I-V characteristic ofthe organic EL element is degraded, the operating point of the drivingtransistor and the organic EL element varies. Thus, even when a samevoltage is applied to the gate of the driving transistor, the sourcepotential of the driving transistor changes. Thereby, the gate-to-sourcevoltage Vgs of the driving transistor changes, and therefore the valueof current flowing through the driving transistor changes. As a result,the value of current flowing through the organic EL element alsochanges, so that the light emission luminance of the organic EL elementchanges.

Further, in a pixel circuit using a polysilicon TFT, in addition to asecular degradation in the I-V characteristic of an organic EL element,transistor characteristics of a driving transistor vary with the passageof time, and the transistor characteristics vary from pixel to pixel dueto variations in a manufacturing process. That is, the transistorcharacteristics of the driving transistor vary between individualpixels. The transistor characteristics include for example the thresholdvoltage Vth of the driving transistor and the mobility μ of asemiconductor thin film forming the channel of the driving transistor(which mobility μ will hereinafter be described simply as the “mobilityμ of the driving transistor”).

When the transistor characteristics of the driving transistor differ ineach pixel, the value of current flowing through the driving transistorvaries in each pixel. Thus, even when a same voltage is applied to thegates of driving transistors in respective pixels, the light emissionluminance of the organic EL element varies between the pixels. As aresult, screen uniformity is impaired.

Accordingly, in order to hold the light emission luminance of theorganic EL element constant without being affected by seculardegradation in the I-V characteristic of the organic EL element orsecular changes in the transistor characteristics of the drivingtransistor, for example, the pixel circuit is provided with variouscorrecting (compensating) functions (see Japanese Patent Laid-Open No.2006-133542, for example).

The correcting functions include for example a function of compensatingfor variations in the characteristic of the organic EL element, afunction of correcting for variations in the threshold voltage Vth ofthe driving transistor, and a function of correcting for variations inthe mobility μ of the driving transistor. Hereinafter, correction forvariations in the threshold voltage Vth of the driving transistor willbe referred to as “threshold value correction,” and correction forvariations in the mobility μ of the driving transistor will be referredto as “mobility correction.”

By thus providing each pixel circuit with the various functions, thelight emission luminance of the organic EL element can be held constantwithout being affected by secular degradation in the I-V characteristicof the organic EL element or secular change in the transistorcharacteristics of the driving transistor. As a result, the displayquality of the organic EL display device can be improved.

SUMMARY OF THE INVENTION

The characteristics of a transistor forming a pixel circuit (whichtransistor may be described as a “pixel transistor”), for example adriving transistor vary with the passage of stress application time,that is, light emission time. Then, a panel current value, specificallythe value of a current flowing through an organic EL element changes. Asa result, the light emission luminance of the organic EL element varies.FIG. 21 shows a current ratio to an initial current value with respectto the emission period of an organic EL element for red light.

In a display device for color display, one pixel is formed by a unit ofthree sub-pixels, which are a sub-pixel emitting red light (R), asub-pixel emitting green light (G), and a sub-pixel emitting blue light(B). The organic EL element has a device structure formed on a substrateon which a pixel transistor and the like are formed with a planarizingfilm interposed between the organic EL element and the substrate (whichdevice structure will be described later in detail).

In such a device structure, the three sub-pixels of RGB forming a unitare disposed so as to be adjacent to each other. Thus, a part (so-calledleakage light) of light emitted by an organic EL element of a certaincolor irradiates adjacent pixel transistors of pixels of the othercolors. Of the pieces of emitted light of RGB, the blue light hashighest energy. Thus, there is mainly a strong effect of the blue lighton the pixel transistors of RG.

Therefore, when the pixel transistor of the adjacent pixel of R, forexample, is irradiated with a part of the blue light emitted by theorganic EL element of B, the characteristics of the pixel transistor ofR change more greatly than when the pixel transistor of R is notirradiated. Because of the variation in the characteristics of the pixeltransistor, as shown in FIG. 21, the current ratio at a time of emissionof the blue light of the organic EL element emitting red light is lowerthan at a time of non-emission of the blue light.

While variation in the characteristics of a pixel transistor due to aneffect of leakage light from the B pixel in the pixel emitting red lighthas been described above, the same is true for the pixel emitting greenlight. A pixel transistor in the pixel of B is also irradiated with thelight emitted by the organic EL element of the pixel of B itself. As aresult, change in the characteristics of the pixel transistor variesfrom color to color.

Thus, when the characteristics of the pixel transistor change due to theeffect of leakage light from an adjacent pixel, and the change in thecharacteristics varies from color to color, a current ratio to aninitial current value with respect to emission time varies from color tocolor. Therefore a problem occurs in that a white balance (balancebetween pieces of emitted light of RGB) is disturbed depending on animage being displayed.

Incidentally, for example a measure of adopting a leakage lightpreventing structure using a rib or the like may be taken to avoid theeffect of light emitted by an adjacent pixel. However, the provision ofa leakage light preventing structure complicates a device structure.

It is accordingly desirable to provide a display device that can reducevariations in changes in the characteristics of the pixel transistorfrom color to color due to the effect of leakage light from an adjacentpixel, a method of laying out light emitting elements in the displaydevice, and an electronic device using the display device.

According to an embodiment of the present invention there is provided adisplay device in which light emitting elements of a plurality of colorsincluding a light emitting element emitting blue light are formed ineach pixel on a substrate on which a transistor is formed for eachsub-pixel, and a plurality of pixels formed with sub-pixels of theplurality of colors as a unit are arranged in a form of a matrix, and

relative positional relation between transistors of sub-pixels ofrespective light emission colors including blue light and a lightemitting section of a light emitting element emitting said blue light islaid out such that distances between the transistors of the sub-pixelsof the respective light emission colors including said blue light andthe light emitting section of the light emitting element emitting saidblue light are equal to each other for the respective colors.

According to another embodiment of the present invention there isprovided a method of laying out light emitting elements, in laying outlight emitting elements in a display device in which light emittingelements of a plurality of colors including a light emitting elementemitting blue light are formed in each pixel on a substrate on which atransistor is formed for each sub-pixel, and a plurality of pixelsformed with sub-pixels of the plurality of colors as a unit are arrangedin a form of a matrix, said method including the step of

laying out relative positional relation between transistors ofsub-pixels of respective light emission colors including blue light anda light emitting section of a light emitting element emitting said bluelight such that distances between the transistors of the sub-pixels ofthe respective light emission colors including said blue light and thelight emitting section of the light emitting element emitting said bluelight are equal to each other for the respective colors.

According to yet another embodiment of the present invention there isprovided a method of laying out light emitting elements, in laying outlight emitting elements in a display device in which light emittingelements of a plurality of colors including a light emitting elementemitting blue light are formed in each pixel on a substrate on which atransistor is formed for each sub-pixel, and a plurality of pixelsformed with sub-pixels of the plurality of colors as a unit are arrangedin a form of a matrix, said method including the step of

laying out relative positional relation between transistors ofsub-pixels of respective light emission colors including blue light anda light emitting section of a light emitting element emitting said bluelight such that a part of the blue light equally irradiates thetransistors of the sub-pixels of the other light emission colors.

Further, according to an embodiment of the present invention there isprovided an electronic device having a display device in which lightemitting elements of a plurality of colors including a light emittingelement emitting blue light are formed in each pixel on a substrate onwhich a transistor is formed for each sub-pixel, and a plurality ofpixels formed with sub-pixels of the plurality of colors as a unit arearranged in a form of a matrix,

wherein relative positional relation between transistors of sub-pixelsof respective light emission colors including blue light and a lightemitting section of a light emitting element emitting said blue light islaid out such that distances between the transistors of the sub-pixelsof the respective light emission colors including said blue light andthe light emitting section of the light emitting element emitting saidblue light are equal to each other for the respective colors.

According to the present invention, variations in changes incharacteristics of the pixel transistor from color to color due to theeffect of leakage light from an adjacent pixel can be reduced. Thereby,a white balance can be maintained without depending on an image beingdisplayed, so that a display image of excellent display quality can beobtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system configuration diagram showing an outline of aconfiguration of an organic EL display device to which embodiments ofthe present invention is applied;

FIG. 2 is a circuit diagram showing an example of circuit configurationof a pixel;

FIG. 3 is a timing waveform chart of assistance in explaining thecircuit operation of the organic EL display device according to thepresent example of application in which a pixel has a 2Tr circuitconfiguration as a basic configuration;

FIGS. 4A, 4B, 4C, and 4D are diagrams (1) of assistance in explainingthe circuit operation of the organic EL display device according to thepresent example of application;

FIGS. 5A, 5B, 5C, and 5D are diagrams (2) of assistance in explainingthe circuit operation of the organic EL display device according to thepresent example of application;

FIG. 6 is a characteristic diagram of assistance in explaining a problemcaused by variations in threshold voltage Vth of a driving transistor;

FIG. 7 is a characteristic diagram of assistance in explaining a problemcaused by variations in mobility μ of the driving transistor;

FIGS. 8A, 8B, and 8C are characteristic diagrams of assistance inexplaining relations between the signal voltage Vsig of a video signaland the drain-to-source current Ids of the driving transistor accordingto whether threshold value correction and mobility correction areperformed or not;

FIG. 9 is a sectional view of a basic sectional structure of a pixel;

FIG. 10 is a diagram showing a state in which a part of light emitted byan organic EL element of a certain color affects adjacent pixeltransistors of other colors;

FIG. 11 is a plan view conceptually showing a layout structure accordingto a first embodiment;

FIG. 12 is a plan view conceptually showing a layout structure accordingto a second embodiment;

FIG. 13 is a plan view conceptually showing a layout structure accordingto a third embodiment;

FIG. 14 is a plan view conceptually showing a layout structure accordingto a fourth embodiment;

FIG. 15 is a circuit diagram showing an example of circuit configurationof a pixel having another configuration;

FIG. 16 is a perspective view of an external appearance of a televisionset to which the embodiments of the present invention is applied;

FIGS. 17A and 17B are perspective views of an external appearance of adigital camera to which the embodiments of the present invention isapplied, FIG. 17A being a perspective view of the digital camera asviewed from a front side, and FIG. 17B being a perspective view of thedigital camera as viewed from a back side;

FIG. 18 is a perspective view of an external appearance of a notebookpersonal computer to which the embodiments of the present invention isapplied;

FIG. 19 is a perspective view of an external appearance of a videocamera to which the embodiments of the present invention is applied;

FIGS. 20A, 20B, 20C, 20D, 20E, 20F, and 20G are diagrams showing anexternal appearance of a portable telephone to which the embodiments ofthe present invention is applied, FIG. 20A being a front view of theportable telephone in an opened state, FIG. 20B being a side view of theportable telephone in the opened state, FIG. 20C being a front view ofthe portable telephone in a closed state, FIG. 20D being a left sideview, FIG. 20E being a right side view, FIG. 20F being a top view, andFIG. 20G being a bottom view; and

FIG. 21 is a diagram showing a current ratio to an initial current valuewith respect to the emission period of an organic EL element for redlight.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will hereinafter bedescribed in detail with reference to the drawings.

[System]

FIG. 1 is a system configuration diagram showing an outline of aconfiguration of an active matrix type display device to which theembodiments of the present invention is applied. Description in thefollowing will be made by taking as an example an active matrix typeorganic EL display device using a current-driven type electroopticelement whose light emission luminance changes according to the value ofa current flowing through the device, for example an organic EL elementas light emitting elements of pixels (pixel circuits).

As shown in FIG. 1, the organic EL display device 10 according to thepresent example of application has a plurality of pixels 20 includinglight emitting elements, a pixel array section 30 in which the pixels 20are arranged two-dimensionally in the form of a matrix, and a drivingsection disposed on the periphery of the pixel array section 30. Thedriving section drives each pixel 20 of the pixel array section 30. Thedriving section includes for example a writing scanning circuit 40, apower supply scanning circuit 50, and a signal outputting circuit 60.

In this case, the organic EL display device 10 is capable of colordisplay. One pixel is formed with a plurality of sub-pixels as a unit,and the individual sub-pixels correspond to the pixel 20. Morespecifically, in a display device for color display, as described above,one pixel is formed with three sub-pixels as a unit, the threesub-pixels being a sub-pixel emitting red light, a sub-pixel emittinggreen light, and a sub-pixel emitting blue light.

However, one pixel is not limited to the combination of sub-pixels ofthree primary colors of RGB, and one pixel can be formed by furtheradding a sub-pixel of one color or sub-pixels of a plurality of colorsto the sub-pixels of the three primary colors. More specifically, forexample, one pixel can be formed by adding a sub-pixel emitting whitelight (W) to improve luminance, or one pixel can be formed by adding atleast one sub-pixel emitting light of a complementary color to expand acolor reproduction range.

The pixel array section 30 has scanning lines 31-1 to 31-m and powersupply lines 32-1 to 32-m arranged for each pixel row along a rowdirection (direction of arrangement of pixels of a pixel row) in anarrangement of the pixels 20 of m rows and n columns. Further, signallines 33-1 to 33-n are arranged for each pixel column along a columndirection (direction of arrangement of pixels of a pixel column).

The scanning lines 31-1 to 31-m are respectively connected to outputterminals for the corresponding rows of the writing scanning circuit 40.The power supply lines 32-1 to 32-m are respectively connected to outputterminals for the corresponding rows of the power supply scanningcircuit 50. The signal lines 33-1 to 33-n are respectively connected tooutput terminals for the corresponding columns of the signal outputtingcircuit 60.

The pixel array section 30 is usually formed on a transparent insulatingsubstrate such as a glass substrate or the like. The organic EL displaydevice 10 thereby has a plane type (flat type) panel structure. Thedriving circuit of each pixel 20 in the pixel array section 30 can beformed using an amorphous silicon TFT or a low-temperature polysiliconTFT. When the low-temperature polysilicon TFT is used, the writingscanning circuit 40, the power supply scanning circuit 50, and thesignal outputting circuit 60 can also be mounted on a display panel(substrate) 70 on which the pixel array section 30 is formed.

The writing scanning circuit 40 is formed by a shift registersequentially shifting (transferring) a start pulse sp in order insynchronism with a clock pulse ck, or the like. At a time of writing avideo signal to the pixels 20 of the pixel array section 30, the writingscanning circuit 40 sequentially supplies a writing scanning signal WS(WS1 to WSm) to the scanning lines 31-1 to 31-m, and thereby scans thepixels 20 of the pixel array section 30 in row units in order(line-sequential scanning).

The power supply scanning circuit 50 is formed by a shift registersequentially shifting (transferring) the start pulse sp in order insynchronism with the clock pulse ck, or the like. The power supplyscanning circuit 50 supplies a power supply potential DS (DS1 to DSm)changing between a first power supply potential Vccp and a second powersupply potential Vini lower than the first power supply potential Vccpto the power supply lines 32-1 to 32-m in synchronism with theline-sequential scanning of the writing scanning circuit 40. Theemission/non-emission of the pixels 20 is controlled by changing thepower supply signal DS to Vccp/Vini.

The signal outputting circuit 60 appropriately selects and outputs oneof the signal voltage Vsig of a video signal corresponding to luminanceinformation supplied from a signal supplying source (not shown) (thesignal voltage Vsig may hereinafter be described simply as a “signalvoltage”) and a reference potential Vofs. The signal voltage Vsig or thereference potential Vofs output from the signal outputting circuit 60 iswritten to the pixels 20 of the pixel array section 30 in row units viathe signal lines 33-1 to 33-n. That is, the signal outputting circuit 60employs a line-sequential writing driving mode in which the signalvoltage Vsig is written in row (line) units.

(Pixel Circuit)

FIG. 2 is a circuit diagram showing an example of circuit configurationof a pixel (pixel circuit) 20.

As shown in FIG. 2, the pixel 20 is formed by a current-driven typeelectrooptic element whose light emission luminance changes according tothe value of a current flowing through the device, for example anorganic EL element 21, and a driving circuit driving the organic ELelement 21. The organic EL element 21 has a cathode electrode connectedto a common power supply line 34 commonly wired to all the pixels 20(so-called solid wiring).

The driving circuit driving the organic EL element 21 includes a drivingtransistor 22, a writing transistor 23, a storage capacitor 24, and anauxiliary capacitance 25. In this case, an N-channel type TFT is used asthe driving transistor 22 and the writing transistor 23. However, thecombination of the conduction type of the driving transistor 22 and theconduction type of the writing transistor 23 is a mere example, and thepresent invention is not limited to the above combination.

Incidentally, when an N-channel type TFT is used as the drivingtransistor 22 and the writing transistor 23, an amorphous silicon (a-Si)process can be used. The use of the a-Si process can reduce the cost ofthe substrate on which the TFTs are made, and in turn reduce the cost ofthe organic EL display device 10. In addition, when the drivingtransistor 22 and the writing transistor 23 are of a same conductiontype, both the transistors 22 and 23 can be made by a same process, andthus contribute to reduction in cost.

The driving transistor 22 has one electrode (source/drain electrode)connected to the anode electrode of the organic EL element 21, and hasanother electrode (drain/source electrode) connected to the power supplyline 32 (power supply lines 32-1 to 32-m).

The writing transistor 23 has one electrode (source/drain electrode)connected to the signal line 33 (signal lines 33-1 to 33-n), and hasanother electrode (drain/source electrode) connected to the gateelectrode of the driving transistor 22. The gate electrode of thewriting transistor 23 is connected to the scanning line 31 (scanninglines 31-1 to 31-m).

In the driving transistor 22 and the writing transistor 23, the oneelectrode refers to metallic wiring electrically connected to asource/drain region, and the other electrode refers to metallic wiringelectrically connected to a drain/source region. Depending on potentialrelation between the one electrode and the other electrode, the oneelectrode is the source electrode or the drain electrode, and the otherelectrode is the drain electrode or the source electrode.

The storage capacitor 24 has one electrode connected to the gateelectrode of the driving transistor 22, and has another electrodeconnected to the other electrode of the driving transistor 22 and theanode electrode of the organic EL element 21.

The auxiliary capacitance 25 has one electrode connected to the anodeelectrode of the organic EL element 21, and has another electrodeconnected to the common power supply line 34. The auxiliary capacitance25 is provided as required in order to supply a lack of capacitance ofthe organic EL element 21 and increase a gain in writing a video signalto the storage capacitor 24. That is, the auxiliary capacitance 25 isnot an essential constituent element, and can be omitted when theequivalent capacitance of the organic EL element 21 is sufficientlylarge.

While the other electrode of the auxiliary capacitance 25 is connectedto the common power supply line 34 in this case, the part to which theother electrode of the auxiliary capacitance 25 is connected is notlimited to the common power supply line 34, and it suffices for the partto which the other electrode of the auxiliary capacitance 25 isconnected to be a node of a fixed potential. Connecting the otherelectrode of the auxiliary capacitance 25 to a fixed potential canachieve the intended objects of supplying a lack of capacitance of theorganic EL element 21 and increase the gain in writing a video signal tothe storage capacitor 24.

In the pixel 20 of the above configuration, the writing transistor 23 isset in a conducting state by responding to a High-active writingscanning signal WS applied from the writing scanning circuit 40 to thegate electrode of the writing transistor 23 via the scanning line 31.Thereby, the writing transistor 23 samples the signal voltage Vsig of avideo signal corresponding to luminance information or the referencepotential Vofs, the signal voltage Vsig or the reference potential Vofsbeing supplied from the signal outputting circuit 60 via the signal line33, and writes the signal voltage Vsig or the reference potential Vofsinto the pixel 20. The written signal voltage Vsig or the writtenreference potential Vofs is applied to the gate electrode of the drivingtransistor 22, and is also retained by the storage capacitor 24.

When the potential DS of the power supply line 32 (power supply lines32-1 to 32-m) is the first power supply potential Vccp, the drivingtransistor 22 operates in a saturation region with the one electrodeserving as a drain electrode and with the other electrode serving as asource electrode. Thereby, the driving transistor 22 is supplied with acurrent from the power supply line 32, and light-emission-drives theorganic EL element 21 by current driving. More specifically, the drivingtransistor 22 operates in the saturation region and thereby supplies adriving current having a current value corresponding to the voltagevalue of the signal voltage Vsig retained by the storage capacitor 24 tothe organic EL element 21 to make the organic EL element 21 emit lightby current-driving the organic EL element 21.

Further, when the power supply potential DS is changed from the firstpower supply potential Vccp to the second power supply potential Vini,the driving transistor 22 operates as a switching transistor with theone electrode serving as a source electrode and with the other electrodeserving as a drain electrode. The driving transistor 22 thereby stopssupplying the driving current to the organic EL element 21 to set theorganic EL element 21 in a non-emission state. That is, the drivingtransistor 22 also has a function of a transistor that controls theemission/non-emission of the organic EL element 21.

A period during which the organic EL element 21 is in a non-emissionstate (non-emission period) is provided by the switching operation ofthe driving transistor 22 to control a ratio (duty) between the emissionperiod and the non-emission period of the organic EL element 21. Thisduty control can reduce an afterimage blur involved in light emission ofpixels over one frame period, and thus achieve more excellent imagequality of a moving image in particular.

In this case, the reference potential Vofs selectively supplied from thesignal outputting circuit 60 via the signal line 33 is a potentialserving as a reference for the signal voltage Vsig of the video signalcorresponding to the luminance information (for example a potentialcorresponding to the black level of the video signal).

Of the first power supply potential Vccp and the second power supplypotential Vini selectively supplied from the power supply scanningcircuit 50 via the power supply line 32, the first power supplypotential Vccp is a power supply potential for supplying the drivingcurrent for light emission driving of the organic EL element 21 to thedriving transistor 22. The second power supply potential Vini is a powersupply potential for applying a reverse bias to the organic EL element21. The second power supply potential Vini is set lower than thereference potential Vofs, or for example, letting Vth be the thresholdvoltage of the driving transistor 22, the second power supply potentialVini is set lower than Vofs−Vth and preferably set sufficiently lowerthan Vofs−Vth.

As is clear from the above description, the pixels 20 in the organic ELdisplay device 10 according to the present example of application has acircuit configuration including two transistors, which are the drivingtransistor 22 and the writing transistor 23, as a basic configuration.However, the basic configuration of the pixels 20 is not limited to the2Tr circuit configuration including the two transistors.

(Circuit Operation of Organic El Display Device)

The circuit operation of the organic EL display device 10 formed withthe pixels 20 of the above-described configuration arrangedtwo-dimensionally in the form of a matrix will next be described withreference to operation explanatory diagrams of FIGS. 4A, 4B, 4C, and 4Dand FIGS. 5A, 5B, 5C, and 5D on the basis of a timing waveform chart ofFIG. 3. Incidentally, in the operation explanatory diagrams of FIGS. 4Ato 4D and FIGS. 5A to 5D, the writing transistor 23 is represented bythe symbol of a switch in order to simplify the drawings.

The timing waveform chart of FIG. 3 shows changes in potential (writingscanning signal) WS of the scanning line 31 (31-1 to 31-m), changes inpotential (power supply potential) DS of the power supply line 32 (32-1to 32-m), and changes in the gate potential Vg and the source potentialVs of the driving transistor 22. In addition, the waveform of the gatepotential Vg is represented by alternate long and short dash lines, andthe waveform of the source potential Vs is represented by a dotted line,so that the two waveforms can be distinguished from each other.

<Emission Period of Preceding Frame>

A period before time t1 in the timing waveform chart of FIG. 3 is anemission period of the organic EL element 21 in a preceding frame(field). In the emission period of the preceding frame, the potential DSof the power supply line 32 is the first power supply potential(hereinafter described as a “high potential”) Vccp, and the writingtransistor 23 is in a non-conducting state.

The driving transistor 22 is designed to operate in the saturationregion at this time. Thereby, as shown in FIG. 4A, a driving current(drain-to-source current) Ids corresponding to the gate-to-sourcevoltage Vgs of the driving transistor 22 is supplied from the powersupply line 32 through the driving transistor 22 to the organic ELelement 21. The organic EL element 21 thus emits light at a luminancecorresponding to the current value of the driving current Ids.

<Threshold Value Correction Preparatory Period>

A new frame (present frame) of line-sequential scanning begins at timet1. As shown in FIG. 4B, the potential DS of the power supply line 32 ischanged from the high potential Vccp to the second power supplypotential (hereinafter described as a “low potential”) Vini sufficientlylower than Vofs−Vth with respect to the reference potential Vofs of thesignal line 33.

Let Vthel be the threshold voltage of the organic EL element 21, andVcath be the potential of the common power supply line 34 (cathodepotential). At this time, when the low potential Vini is set to beVini<Vthel+Vcath, the source potential Vs of the driving transistor 22becomes substantially equal to the low potential Vini, and thus theorganic EL element 21 is set in a reverse-biased state and quenched.

Next, at time t2, the potential WS of the scanning line 31 makes atransition from a low potential side to a high potential side, wherebythe writing transistor 23 is set in a conducting state, as shown in FIG.4C. At this time, because the reference potential Vofs is supplied fromthe signal outputting circuit 60 to the signal line 33, the gatepotential Vg of the driving transistor 22 becomes the referencepotential Vofs. The source potential Vs of the driving transistor 22 isthe potential Vini, which is sufficiently lower than the referencepotential Vofs.

At this time, the gate-to-source voltage Vgs of the driving transistor22 is Vofs−Vini. A threshold value correcting process to be describedlater cannot be performed unless Vofs−Vini is larger than the thresholdvoltage Vth of the driving transistor 22. Therefore a potential relationsuch that Vofs−Vini>Vth needs to be set.

The process of thus initializing the gate potential Vg and the sourcepotential Vs of the driving transistor 22 by fixing (establishing) thegate potential Vg of the driving transistor 22 to the referencepotential Vofs and the source potential Vs of the driving transistor 22to the low potential Vini is the preparatory (threshold value correctionpreparatory) process before a threshold value correcting process to bedescribed later is performed. Thus, the reference potential Vofs and thelow potential Vini are respective initializing potentials for the gatepotential Vg and the source potential Vs of the driving transistor 22.

<Threshold Value Correcting Period>

Next, when the potential DS of the power supply line 32 is changed fromthe low potential Vini to the high potential Vccp at time t3 as shown inFIG. 4D, a threshold value correcting process begins in a state of thegate potential Vg of the driving transistor 22 being retained. That is,the source potential Vs of the driving transistor 22 starts risingtoward a potential obtained by subtracting the threshold voltage Vth ofthe driving transistor 22 from the gate potential Vg.

In this case, for convenience, with the initializing potential Vofs ofthe gate electrode of the driving transistor 22 as a reference, theprocess of changing the source potential Vs toward the potentialobtained by subtracting the threshold voltage Vth of the drivingtransistor 22 from the initializing potential Vofs is referred to as athreshold value correcting process. As the threshold value correctingprocess progresses, the gate-to-source voltage Vgs of the drivingtransistor 22 eventually converges to the threshold voltage Vth of thedriving transistor 22. A voltage corresponding to the threshold voltageVth is retained by the storage capacitor 24.

Incidentally, suppose that in a period in which the threshold valuecorrecting process is performed (threshold value correcting period), inorder for a current to flow only to the side of the storage capacitor 24and not to flow to the side of the organic EL element 21, the potentialVcath of the common power supply line 34 is set such that the organic ELelement 21 is in a cutoff state.

Next, the potential WS of the scanning line 31 makes a transition to thelow potential side at time t4, whereby the writing transistor 23 is setin a non-conducting state as shown in FIG. 5A. At this time, the gateelectrode of the driving transistor 22 is electrically disconnected fromthe signal line 33, and is thereby set in a floating state. However,because the gate-to-source voltage Vgs is equal to the threshold voltageVth of the driving transistor 22, the driving transistor 22 is in acutoff state. Therefore the drain-to-source current Ids does not flowthrough the driving transistor 22.

<Signal Writing and Mobility Correcting Period>

Next, at time t5, as shown in FIG. 5B, the potential of the signal line33 is changed from the reference potential Vofs to the signal voltageVsig of the video signal. Then, at time t6, the potential WS of thescanning line 31 makes a transition to the high potential side. Thereby,as shown in FIG. 5C, the writing transistor 23 is set in a conductingstate to sample the signal voltage Vsig of the video signal and writethe signal voltage Vsig into the pixel 20.

As a result of the writing of the signal voltage Vsig by the writingtransistor 23, the gate potential Vg of the driving transistor 22becomes the signal voltage Vsig. At a time of driving the drivingtransistor 22 by the signal voltage Vsig of the video signal, thethreshold voltage Vth of the driving transistor 22 is cancelled out bythe voltage retained by the storage capacitor 24 and corresponding tothe threshold voltage Vth. Details of principles of this threshold valuecancellation will be described later.

At this time, the organic EL element 21 is in a cutoff state (state ofhigh impedance). Thus, a current (drain-to-source current Ids) flowingfrom the power supply line 32 to the driving transistor 22 according tothe signal voltage Vsig of the video signal flows into the auxiliarycapacitance 25. Thus, the charging of the auxiliary capacitance 25 isstarted.

The charging of the auxiliary capacitance 25 raises the source potentialVs of the driving transistor 22 with the passage of time. At this time,a variation in the threshold voltage Vth of the driving transistor 22 ineach pixel is already cancelled, and the drain-to-source current Ids ofthe driving transistor 22 is dependent on mobility μ of the drivingtransistor 22.

Suppose that in this case, a ratio of the gate-to-source voltage Vgsretained by the storage capacitor 24 to the signal voltage Vsig of thevideo signal, that is, a writing gain is one (ideal value). Then, thesource potential Vs of the driving transistor 22 rises to a potentialVofs−Vth+ΔV, whereby the gate-to-source voltage Vgs of the drivingtransistor 22 is Vsig−Vofs+Vth−ΔV.

That is, the rise ΔV in the source potential Vs of the drivingtransistor 22 is subtracted from the voltage (Vsig−Vofs+Vth) retained bythe storage capacitor 24, or in other words, the rise ΔV in the sourcepotential Vs of the driving transistor 22 acts to discharge the chargestored in the storage capacitor 24, so that a negative feedback isapplied. Thus, the rise ΔV in the source potential Vs is a feedbackamount of the negative feedback.

Thus applying a negative feedback to the gate-to-source voltage Vgs bythe feedback amount ΔV corresponding to the drain-to-source current Idsflowing through the driving transistor 22 can cancel out the dependenceof the drain-to-source current Ids of the driving transistor 22 on themobility μ. This canceling process is a mobility correcting process thatcorrects a variation in the mobility μ of the driving transistor 22 ineach pixel.

More specifically, the higher the signal amplitude Vin (=Vsig−Vofs) ofthe video signal written to the gate electrode of the driving transistor22, the larger the drain-to-source current Ids, and thus the greater theabsolute value of the feedback amount ΔV of the negative feedback.Therefore the mobility correcting process is performed according tolight emission luminance level.

In addition, when the signal amplitude Vin of the video signal is fixed,the higher the mobility μ of the driving transistor 22, the greater theabsolute value of the feedback amount ΔV of the negative feedback, sothat a variation in mobility μ in each pixel can be eliminated.Therefore the feedback amount ΔV of the negative feedback can also besaid to be a correction amount of mobility correction. Details ofprinciples of the mobility correction will be described later.

<Emission Period>

Next, the potential WS of the scanning line 31 makes a transition to thelow potential side at time t7, whereby the writing transistor 23 is setin a non-conducting state as shown in FIG. 5D. Thereby, the gateelectrode of the driving transistor 22 is electrically disconnected fromthe signal line 33, and is thus set in a floating state.

When the gate electrode of the driving transistor 22 is in a floatingstate, the gate potential Vg of the driving transistor 22 varies in sucha manner as to be interlocked with variation in the source potential Vsof the driving transistor 22 because the storage capacitor 24 isconnected between the gate and the source of the driving transistor 22.The operation of the gate potential Vg of the driving transistor 22 thusvarying in such a manner as to be interlocked with variation in thesource potential Vs of the driving transistor 22 is a bootstrapoperation by the storage capacitor 24.

The gate electrode of the driving transistor 22 is set in a floatingstate, and at the same time, the drain-to-source current Ids of thedriving transistor 22 starts to flow to the organic EL element 21.Thereby the anode potential of the organic EL element 21 rises accordingto the current Ids.

When the anode potential of the organic EL element 21 exceedsVthel+Vcath, the driving current starts flowing through the organic ELelement 21, and therefore the organic EL element 21 starts emittinglight. A rise in the anode potential of the organic EL element 21 isnone other than a rise in the source potential Vs of the drivingtransistor 22. When the source potential Vs of the driving transistor 22rises, the gate potential Vg of the driving transistor 22 is also raisedin an interlocked manner by the bootstrap operation of the storagecapacitor 24.

At this time, supposing that a bootstrap gain is one (ideal value), theamount of the rise in the gate potential Vg is equal to the amount ofthe rise in the source potential Vs. Therefore the gate-to-sourcevoltage Vgs of the driving transistor 22 during the emission period ismaintained at a fixed level Vsig−Vofs+Vth−ΔV. Then, at time t8, thepotential of the signal line 33 is changed from the signal voltage Vsigof the video signal to the reference potential Vofs.

The respective process operations of the threshold value correctionpreparation, the threshold value correction, the writing of the signalvoltage Vsig (signal writing), and the mobility correction in the seriesof circuit operations described above are performed in one horizontalscanning period (1H). The respective process operations of the signalwriting and the mobility correction are performed in parallel with eachother in a period from time t6 to time t7.

(Principles of Threshold Value Correction)

Principles of threshold value cancellation (that is, threshold valuecorrection) of the driving transistor 22 will be described in thefollowing. The driving transistor 22 is designed to operate in asaturation region, and thus operates as a constant-current source.Thereby a constant drain-to-source current (driving current) Ids givenby the following Equation (1) is supplied from the driving transistor 22to the organic EL element 21.

Ids=(½)·μ(W/L)Cox(Vgs−Vth)²  (1)

where W is the channel width of the driving transistor 22, L is thechannel length of the driving transistor 22, and Cox is gate capacitanceper unit area.

FIG. 6 shows a characteristic of the drain-to-source current Ids of thedriving transistor 22 versus the gate-to-source voltage Vgs of thedriving transistor 22.

As shown in this characteristic diagram, without the process ofcancelling variation in the threshold voltage Vth of the drivingtransistor 22 in each pixel, when the threshold voltage Vth is Vth1, thedrain-to-source current Ids corresponding to the gate-to-source voltageVgs is Ids1.

On the other hand, when the threshold voltage Vth is Vth2 (Vth2>Vth1),the drain-to-source current Ids corresponding to the same gate-to-sourcevoltage Vgs is Ids2 (Ids2<Ids1). That is, when the threshold voltage Vthof the driving transistor 22 varies, the drain-to-source current Idsvaries even if the gate-to-source voltage Vgs is constant.

On the other hand, in the pixel (pixel circuit) 20 of theabove-described configuration, the gate-to-source voltage Vgs of thedriving transistor 22 at the time of light emission is Vsig−Vofs+Vth−ΔV,as described above. Thus, when this is substituted into Equation (1),the drain-to-source current Ids is expressed by the following Equation(2).

Ids=(½)·μ(W/L)Cox(Vsig−Vofs−ΔV)²  (2)

That is, the term of the threshold voltage Vth of the driving transistor22 is cancelled, and therefore the drain-to-source current Ids suppliedfrom the driving transistor 22 to the organic EL element 21 is notdependent on the threshold voltage Vth of the driving transistor 22. Asa result, even when the threshold voltage Vth of the driving transistor22 varies in each pixel due to variations in a process of manufacturingthe driving transistor 22 or a secular change in the driving transistor22, the drain-to-source current Ids does not vary. Therefore the lightemission luminance of the organic EL element 21 can be held constant.

(Principles of Mobility Correction)

Principles of the mobility correction of the driving transistor 22 willnext be described. FIG. 7 shows characteristic curves in a state inwhich a pixel A whose driving transistor 22 has a relatively highmobility μ and a pixel B whose driving transistor 22 has a relativelylow mobility μ are compared with each other. When the driving transistor22 is formed by a polysilicon thin film transistor or the like, themobility μ inevitably varies between pixels such as the pixel A and thepixel B.

Consideration will be given to a case where for example both pixels Aand B have a signal amplitude Vin (=Vsig−Vofs) at a same level writtento the gate electrodes of the driving transistors 22 with the mobility μvarying between the pixel A and the pixel B. In this case, when nocorrection is made for the mobility μ, a large difference occurs betweena drain-to-source current Ids1′ flowing in the pixel A of high mobilityμ and a drain-to-source current Ids2′ flowing in the pixel B of lowmobility μ. A large difference in drain-to-source current Ids thusoccurring between pixels due to a variation in mobility μ in each pixelimpairs the uniformity of the screen.

As is clear from the above-described Equation (1) as a transistorcharacteristic equation, when the mobility μ is high, thedrain-to-source current Ids is increased. Hence, the higher the mobilityμ, the larger the feedback amount ΔV of negative feedback. As shown inFIG. 7, the feedback amount ΔV1 of the pixel A of high mobility μ islarger than the feedback amount ΔV2 of the pixel B of low mobility.

Accordingly, the mobility correcting process applies a negative feedbackto the gate-to-source voltage Vgs by a feedback amount ΔV correspondingto the drain-to-source current Ids of the driving transistor 22. Therebya larger amount of negative feedback is applied as the mobility μ isincreased. As a result, variations in mobility μ in each pixel can besuppressed.

Specifically, when a correction of the feedback amount ΔV1 is applied inthe pixel A of high mobility μ, the drain-to-source current Ids fallsgreatly from Ids1′ to Ids1. On the other hand, because the feedbackamount ΔV2 of the pixel B of low mobility μ is small, thedrain-to-source current Ids falls from Ids2′ to Ids2, and thus does notfall so greatly. Consequently, the drain-to-source current Ids1 of thepixel A and the drain-to-source current Ids2 of the pixel B becomesubstantially equal to each other. Therefore variations in mobility μ ineach pixel are corrected.

Summarizing the above, when there are a pixel A and a pixel B ofdifferent mobilities p, the feedback amount ΔV1 of the pixel A of highmobility μ is larger than the feedback amount ΔV2 of the pixel B of lowmobility p. That is, the higher the mobility μ of a pixel, the largerthe feedback amount ΔV, and the larger the amount of decrease indrain-to-source current Ids.

Thus, by applying a negative feedback to the gate-to-source voltage Vgsby a feedback amount ΔV corresponding to the drain-to-source current Idsof the driving transistor 22, the current values of drain-to-sourcecurrents Ids in pixels of different mobilities μ are uniformized. As aresult, variations in mobility μ in each pixel can be corrected. Thatis, the process of applying a negative feedback to the gate-to-sourcevoltage Vgs of the driving transistor 22 by a feedback amount ΔVcorresponding to the current (drain-to-source current Ids) flowingthrough the driving transistor 22 is the mobility correcting process.

Relations between the signal voltage Vsig of the video signal and thedrain-to-source current Ids of the driving transistor 22 according towhether threshold value correction and mobility correction are performedor not in the pixel (pixel circuit) 20 shown in FIG. 2 will be describedin the following with reference to FIGS. 8A, 8B, and 8C.

FIG. 8A represents a case where neither threshold value correction normobility correction is performed; FIG. 8B represents a case wheremobility correction is not performed and only threshold value correctionis performed; and FIG. 8C represents a case where threshold valuecorrection and mobility correction are both performed. As shown in FIG.8A, when neither threshold value correction nor mobility correction isperformed, variations in threshold voltage Vth and mobility μ in thepixels A and B cause a large difference in drain-to-source current Idsbetween the pixels A and B.

On the other hand, when only threshold value correction is performed, asshown in FIG. 8B, variations in drain-to-source current Ids can bereduced to some extent, but a difference in drain-to-source current Idsbetween the pixels A and B due to variations in mobility a in the pixelsA and B remains. By performing both threshold value correction andmobility correction, as shown in FIG. 8C, a difference indrain-to-source current Ids between the pixels A and B due to variationsin threshold voltage Vth and mobility μ in the pixels A and B can besubstantially eliminated. Thus, no variations in luminance of theorganic EL element 21 occur at any gradation, so that a display image ofexcellent quality can be obtained.

In addition, the pixel 20 shown in FIG. 2 can provide the followingaction and effect by having the function of bootstrap operation by thestorage capacitor 24 as described above in addition to the respectivecorrecting functions of threshold value correction and mobilitycorrection.

Even when the source potential Vs of the driving transistor 22 ischanged with a secular change in I-V characteristic of the organic ELelement 21, the gate-to-source voltage Vgs of the driving transistor 22can be held constant by the bootstrap operation of the storage capacitor24. Therefore the current flowing through the organic EL element 21 isunchanged and constant. As a result, the light emission luminance of theorganic EL element 21 is also held constant. Thus, even when a secularchange in I-V characteristic of the organic EL element 21 occurs, imagedisplay without luminance degradation attendant on the secular change inI-V characteristic of the organic EL element 21 can be achieved.

(Basic Sectional Structure of Pixel)

A basic sectional structure of the pixel (sub-pixel) 20 will bedescribed in the following. FIG. 9 is a sectional view of a basicsectional structure of the pixel 20.

As shown in FIG. 9, a driving circuit including a pixel transistor 220such as the driving transistor 22 or the like is formed on a glasssubstrate 201. In this case, of constituent elements of the drivingcircuit, only the driving transistor 22 is shown in the figure, and theother constituent elements are omitted. Various pieces of wiring 203 areformed on the glass substrate 201 on which the driving circuit isformed, with an insulating film 202 interposed between the glasssubstrate 201 and the wiring 203, and a planarizing film (resin film)204 is formed on the wiring 203. An anode electrode 205 made of a metalor the like is formed on the planarizing film 204. The anode electrode205 is brought into contact with predetermined pieces of wiring 203.

A window insulating film 206 is formed on the anode electrode 205.Organic EL elements 21R, 21G, and 21B of RGB are provided in concaveparts 206A of the window insulating film 206. Thereby, the openings ofthe concave parts 206A where the organic EL elements 21R, 21G, and 21Bare provided become windows, and light of each color is emitted throughthe windows. That is, the windows of the window insulating film 206 canalso be said to be the light emitting parts of the organic EL elements21R, 21G, and 21B. A cathode electrode 207 made of a transparentconductive film or the like is formed on the window insulating film 206as an electrode common to all pixels.

Though details of the organic EL elements 21R, 21G, and 21B are notshown in FIG. 9, the organic EL elements 21R, 21G, and 21B are of astructure having an organic layer interposed between the anode electrode205 and the cathode electrode 207. The organic layer is formed bysequentially depositing a hole transporting layer/hole injection layer,a light emitting layer, an electron transporting layer, and an electroninjection layer on the anode electrode 205. Under the current driving ofthe driving transistor 22 in FIG. 2, a current flows from the drivingtransistor 22 through the anode electrode 205 to the organic layer, sothat light is emitted at a time of recombination of electrons and holesin the light emitting layer within the organic layer.

The driving transistor 220 is composed of a gate electrode 221,source/drain regions 223 and 224 provided on both end parts of asemiconductor layer 222, and a channel forming region 225 as a partopposed to the gate electrode 221 of the semiconductor layer 222. Thesource/drain region 223 is electrically connected to the anode electrode205 of the organic EL element 21 via a contact hole.

After the organic EL elements 21R, 21G, and 21B are formed in a pixelunit on the glass substrate 201 via the insulating film 202, theplanarizing film 204, and the window insulating film 206, a sealingsubstrate 210 is bonded by an adhesive 209 via an inorganic sealing film208 of SiN or the like. The display panel 70 is formed by sealing theorganic EL elements 21R, 21G, and 21B by the sealing substrate 210.

As described above, in a device structure of the pixel 20 formed via theplanarizing film 204 on the glass substrate 201 on which the pixelcircuit including the pixel transistor 220 is formed, three pixels(sub-pixels) 20R, 20G, and 20B of RGB as a unit are disposed so as to beadjacent to each other. When the organic EL elements are a surface lightsource, for example, light emitted from an organic EL element of acertain color irradiates the periphery with a uniform distribution, asshown in FIG. 10. As a result, a part (leakage light) of the lightemitted by the organic EL element of the certain color irradiatesadjacent pixel transistors of other colors. At this time, blue lighthaving a highest energy, in particular, has a strong effect on the pixeltransistors of R and G.

As is clear from FIG. 10, also in the pixel of B, light emitted by theorganic EL element 21B of the pixel of B itself irradiates the pixeltransistor in the pixel of B. As described earlier, when the pixeltransistors of RGB are irradiated with a part of blue light emitted bythe organic EL element 21B of B, the characteristics of the pixeltransistors change more greatly than when the pixel transistors are notirradiated (see FIG. 21). When changes in characteristics of the pixeltransistors vary from color to color, a current ratio to an initialcurrent value with respect to light emission time varies from color tocolor. Therefore a white balance is disturbed depending on an imagebeing displayed.

[Characteristic Parts of Present Embodiment]

Accordingly, the present embodiment employs a structure in whichrelative positional relation between the pixel transistors of the pixels(sub-pixels) 20R, 20G, and 20B of respective light emission colorsincluding blue light and the light emitting section of the organic ELelement 21B emitting the blue light is laid out such that distancesbetween the pixel transistors of the pixels (sub-pixels) 20R, 20G, and20B and the light emitting section of the organic EL element 21B areequal to each other for the respective colors. The light emittingsection of the organic EL element 21 refers to a part where light isemitted through a window of the window insulating film 206 describedearlier. Concrete embodiments of the layout structure will be describedin the following.

First Embodiment

FIG. 11 is a plan view conceptually showing a layout structure accordingto a first embodiment. As shown in FIG. 11, the organic EL elements 21R,21G, and 21B of RGB are arranged so as to be adjacent to each otherwithin a pixel forming region 230 of a square shape, for example, as aregion for forming one pixel.

Specifically, the respective windows (light emitting sections) 231R and231G of the organic EL elements 21R and 21G of RG are formed side byside in a shape close to a square on one side (lower side in the presentexample) in a column direction (direction of arrangement of pixels of apixel row) of the pixel forming region 230. In addition, the window 231Bof the organic EL element 21B of B is formed in a rectangular shape overthe widths of the windows 231R and 231G on another side (upper side inthe present example) in the column direction of the pixel forming region230.

In addition, within the pixel forming region 230, the respective pixeltransistors (for example the driving transistor 22 in FIG. 2) 220R,220G, and 220B of the pixels 20R, 20G, and 20B of RGB are laid out atpositions separated at a fixed distance from the window 231B of theorganic EL element 21B along a direction of length of the window 231B.Suppose in this case that the organic EL element 21B emitting blue lightis a surface light source. When the organic EL element 21B of B is asurface light source, light emission points of the organic EL element21B with respect to the pixel transistors 220R, 220G, and 220B can beconsidered to be three points Pr, Pg, and Pb at a shortest distance fromthe pixel transistors and on a center line O of the window 231B.

As a result, when the organic EL element 21B of B is a surface lightsource, distances between the pixel transistors 220R, 220G, and 220B andthe light emission points Pr, Pg, and Pb of the organic EL element 21Bof B are equal to each other for the respective colors. Incidentally,suppose that being equal in the above expression includes some error andsome difference in distance due to fine adjustment. With such a layoutstructure, a part of blue light emitted by the organic EL element 21B ofB equally irradiates the pixel transistors 220R, 220G, and 220B.

Thereby, when irradiated with leakage light from the pixel 20B of B, thepixel transistors 220R, 220G, and 220B uniformly vary the transistorcharacteristics thereof, so that variations in characteristic changesfrom color to color can be reduced. As a result, a white balance can bemaintained without depending on an image being displayed, so that adisplay image of excellent display quality can be obtained.

Second Embodiment

FIG. 12 is a plan view conceptually showing a layout structure accordingto a second embodiment. In FIG. 12, same parts as in FIG. 11 areidentified by the same reference numerals. As is clear from FIG. 12, theshapes and arrangement relation of the respective windows (lightemitting sections) 231R, 231G, and 231B of organic EL elements 21R, 21G,and 21B are the same as in the first embodiment.

On the other hand, whereas a surface light source is used as the organicEL element 21B emitting blue light in the first embodiment, a pointlight source is used as the organic EL element 21B in the present secondembodiment. Because the organic EL element 21B is a point light source,the light emission point P of the organic EL element 21B can beconsidered to be the center of the window 231B of the organic EL element21B.

When the light emission point P of the organic EL element 21B is thecenter of the window 231B, in the layout of the pixel transistors 220R,220G, and 220B in the first embodiment, the position of the pixeltransistor 220B is at a shortest distance from the light emission pointP. Accordingly, in the second embodiment, the pixel transistor 220B isdisposed at a position separated from the window 231B as compared withthe other pixel transistors 220R and 220G so that distances of the pixeltransistors 220R, 220G, and 220B from the light emission point P areequal to each other for the respective colors.

Thus, when the organic EL element 21B is a point light source, the pixeltransistors 220R, 220G, and 220B are arranged such that distances of thepixel transistors 220R, 220G, and 220B from the light emission point Pof the organic EL element 21B are equal to each other for the respectivecolors. Thereby similar action and effects to those of the firstembodiment can be obtained. That is, a part of blue light emitted by theorganic EL element 21B of B equally irradiates the pixel transistors220R, 220G, and 220B, and thus the characteristics of the pixeltransistors 220R, 220G, and 220B vary uniformly. Therefore variations incharacteristic changes from color to color can be reduced. As a result,a white balance can be maintained without depending on an image beingdisplayed, so that a display image of excellent display quality can beobtained.

Third Embodiment

FIG. 13 is a plan view conceptually showing a layout structure accordingto a third embodiment. In FIG. 13, same parts as in FIG. 11 areidentified by the same reference numerals.

In the present third embodiment, in a pixel arrangement of pixels(sub-pixels) 20R, 20G, and 20B of RGB repetitively arranged along a rowdirection (direction of arrangement of pixels of a pixel row), threepixels 20G, 20B, and 20R with the pixel 20B in the middle form a unit ofone pixel. The organic EL element 21B may be a surface light source or apoint light source.

Description in the following will be made of a case where a point lightsource is used as the organic EL element 21B. When the organic ELelement 21B is a point light source, the light emission point P of theorganic EL element 21B can be considered to be the center of the window231B of the organic EL element 21B. Blue light emitted from the lightemission point P of the organic EL element 21B can be considered toirradiate the periphery with a uniform distribution having the lightemission point P as a center.

Suppose in this case that the respective pixel transistors 220G and 220Rof the pixels 20G and 20R of G and R are arranged on a center line Opassing through the light emission point P, for example. At this time,the pixel transistor 220B of the pixel 20B of B is disposed at aposition distant from the center line O so that distances of the pixeltransistors 220R, 220G, and 220B from the light emission point P areequal to each other for the respective colors.

Similar action and effects to those of the first embodiment can beobtained by arranging the pixel transistors 220R, 220G, and 220B suchthat the distances of the pixel transistors 220R, 220G, and 220B fromthe light emission point P of the organic EL element 21B are equal toeach other for the respective colors. That is, a part of blue lightemitted by the organic EL element 21B of B equally irradiates the pixeltransistors 220R, 220G, and 220B, and thus variations in characteristicchanges of the pixel transistors 220R, 220G, and 220B from color tocolor can be reduced. As a result, a white balance can be maintainedwithout depending on an image being displayed, so that a display imageof excellent display quality can be obtained.

Incidentally, while the present third embodiment has been describedassuming that a point light source is used as the organic EL element21B, the third embodiment is also applicable to a case where a surfacelight source is used as the organic EL element 21B. In short, itsuffices for the light emitting section of the organic EL element 21Band the pixel transistors 220R, 220G, and 220B to be in a layoutrelation such that a part of blue light emitted by the organic ELelement 21B of B equally irradiates the pixel transistors 220R, 220G,and 220B.

Fourth Embodiment

FIG. 14 is a plan view conceptually showing a layout structure accordingto a fourth embodiment. In FIG. 14, same parts as in FIG. 11 areidentified by the same reference numerals.

In the present fourth embodiment, in a pixel arrangement of pixels 20R,20G, and 20B of RGB repetitively arranged along a row direction, lightemitting sections, that is, windows 231R, 231G, and 231B of organic ELelements 21R, 21G, and 21B are formed obliquely. That is, the windows231R, 231G, and 231B are formed in a state of being inclined at apredetermined angle to a column direction (direction of arrangement ofpixels of a pixel column). On the other hand, the respective pixeltransistors 220R, 220G, and 220B of the pixels 20R, 20G, and 20B arearranged on a center line O passing through the centers of the windows231R, 231G, and 231B, for example.

Suppose that for example a surface light source is used as the organicEL element 21B of B. When the organic EL element 21B is a surface lightsource, as shown in FIG. 14, blue light emitted from the organic ELelement 21B irradiates the periphery of the window 231G with a uniformdistribution from the window 231G. At this time, because the windows231R, 231G, and 231B are inclined, an amount of irradiation of theadjacent pixels 20R and 20G with the blue light emitted by the organicEL element 21B of B increases. Thereby the amounts of irradiation of thepixel transistors 220R and 220G of the adjacent pixels 20R and 20G withthe blue light can be made closer to an amount of irradiation of thepixel transistor 220B of the pixel 20B.

By thus making the amounts of irradiation of the pixel transistors 220R,220G, and 220B with a part of the blue light emitted by the organic ELelement 21B of B close to each other, variations in characteristicchanges from color to color can be reduced. As a result, a white balancecan be maintained without depending on an image being displayed, so thata display image of excellent display quality can be obtained.

Incidentally, while in the fourth embodiment, the respective pixeltransistors 220R, 220G, and 220B of the pixels 20R, 20G, and 20B arearranged on the center line O passing through the centers of the windows231R, 231G, and 231B, the embodiments of the present invention is notlimited to this layout. For example, with respect to the irradiationdistribution of the blue light shown in FIG. 14, the pixel transistor220G is shifted in a downward direction of the figure, and the pixeltransistor 220R is shifted in an upward direction of the figure. Thus,the amounts of irradiation of the pixel transistors 220R and 220G of theadjacent pixels 20R and 20G with the blue light can be made even closerto the amount of irradiation of the pixel transistor 220B of the pixel20B.

[Examples of Modification]

While the foregoing embodiments have been described by taking as anexample a case where the driving circuit of the organic EL element 21has a 2Tr circuit configuration composed of two transistors, which arethe driving transistor 22 and the writing transistor 23, as a basicconfiguration, as shown in FIG. 2, the present invention is not limitedto application to this circuit configuration.

As an example, as shown in FIG. 15, a pixel 20′ is known which pixelhas, as a basic configuration, a 5Tr circuit configuration composed offive transistors, which are a light emission controlling transistor 28and two switching transistors 29 and 30 in addition to a drivingtransistor 22 and a writing transistor 23 (see Japanese Patent Laid-OpenNo. 2005-345722). In this case, a Pch transistor is used as the lightemission controlling transistor 28, and an Nch transistor is used as theswitching transistors 29 and 30. However, the combination of theconduction types of these transistors is arbitrary.

The light emission controlling transistor 28 is connected in series withthe driving transistor 22. The light emission controlling transistor 28selectively supplies a high potential Vccp to the driving transistor 22,thereby controlling emission/non-emission of an organic EL element 21.The switching transistor 29 selectively supplies a reference potentialVofs to the gate electrode of the driving transistor 22, therebyinitializing the gate potential Vg of the driving transistor 22 to thereference potential Vofs. The switching transistor 30 selectivelysupplies a low potential Vini to the source electrode of the drivingtransistor 22, thereby initializing the source potential Vs of thedriving transistor 22 to the low potential Vini.

While the 5Tr circuit configuration has been taken above as an exampleof another pixel configuration, various pixel configurations areconceivable, including for example a configuration in which theswitching transistor 29 is omitted by supplying the reference potentialVofs through a signal line 33 and writing the reference potential Vofsby the writing transistor 23.

In addition, while the foregoing embodiments have been described bytaking as an example a case where the present invention is applied to anorganic EL display device using an organic EL element as an electroopticelement of the pixel 20, the present invention is not limited to thisapplication example. Specifically, the present invention is applicableto display devices in general using a current-driven type electroopticelement (light emitting element) whose light emission luminance changesaccording to the value of a current flowing through the device, such asan inorganic EL element, a LED element, a semiconductor laser element orthe like.

[Examples of Application]

A display device according to an embodiment of the present invention isapplicable to display devices of electronic devices in all fields thatdisplay a video signal input thereto or a video signal generated thereinas an image or video. For example, a display device according to anembodiment of the present invention is applicable to display devices ofvarious electronic devices shown in FIGS. 16 to 20G, such for example asdigital cameras, notebook personal computers, portable terminal devicessuch as portable telephones and the like, and video cameras.

By thus using a display device according to an embodiment of the presentinvention as display devices of electronic devices in all fields,high-quality image display can be made in various electronic devices.Specifically, as is clear from the description of the foregoingembodiments, a display device according to an embodiment of the presentinvention can provide a high-quality display image because the displaydevice can reduce variations in characteristic changes of the pixeltransistors from color to color, and maintain a white balance withoutdepending on the display image.

A display device according to an embodiment of the present inventionincludes a display device in the form of a sealed module. For example, adisplay module formed by attaching a counter part such as a transparentglass or the like to a pixel array section corresponds to a displaydevice in the form of a sealed module. This transparent counter part maybe provided with a color filter, a protective film or the like, and alight shielding film as described above. Incidentally, the displaymodule may be provided with a circuit part, an FPC (Flexible PrintedCircuit), or the like for externally inputting or outputting a signaland the like to the pixel array section.

Concrete examples of electronic devices to which the present inventionis applied will be described in the following.

FIG. 16 is a perspective view of an external appearance of a televisionset to which the present invention is applied. The television setaccording to the present example of application includes a video displayscreen part 101 composed of a front panel 102, a filter glass 103 andthe like, and is fabricated using a display device according to anembodiment of the present invention as the video display screen part101.

FIGS. 17A and 17B are perspective views of an external appearance of adigital camera to which the present invention is applied. FIG. 17A is aperspective view of the digital camera as viewed from a front side, andFIG. 17B is a perspective view of the digital camera as viewed from aback side. The digital camera according to the present example ofapplication includes a light emitting part 111 for flashlight, a displaypart 112, a menu switch 113, a shutter button 114, and the like. Thedigital camera is fabricated using a display device according to anembodiment of the present invention as the display part 112.

FIG. 18 is a perspective view of an external appearance of a notebookpersonal computer to which the present invention is applied. Thenotebook personal computer according to the present example ofapplication includes a keyboard 122 operated to input characters and thelike, a display part 123 for displaying an image, and the like in a mainunit 121. The notebook personal computer is fabricated using a displaydevice according to an embodiment of the present invention as thedisplay part 123.

FIG. 19 is a perspective view of an external appearance of a videocamera to which the present invention is applied. The video cameraaccording to the present example of application includes a main unit131, a lens 132 for taking a subject in a side surface facing frontward,a start/stop switch 133 at a time of picture taking, a display part 134,and the like. The video camera is fabricated using a display deviceaccording to an embodiment of the present invention as the display part134.

FIGS. 20A, 20B, 20C, 20D, 20E, 20F, and 20G are diagrams showing anexternal appearance of a portable terminal device, for example aportable telephone to which the present invention is applied. FIG. 20Ais a front view of the portable telephone in an opened state, FIG. 20Bis a side view of the portable telephone in the opened state, FIG. 20Cis a front view of the portable telephone in a closed state, FIG. 20D isa left side view, FIG. 20E is a right side view, FIG. 20F is a top view,and FIG. 20G is a bottom view. The portable telephone according to thepresent example of application includes an upper side casing 141, alower side casing 142, a coupling part (a hinge part in this case) 143,a display 144, a sub-display 145, a picture light 146, a camera 147, andthe like. The portable telephone according to the present example ofapplication is fabricated using a display device according to anembodiment of the present invention as the display 144 and thesub-display 145.

The present application contains subject matter related to thatdisclosed in Japanese Priority Patent Application JP 2008-132854 filedin the Japan Patent Office on May 21, 2008, the entire content of whichis hereby incorporated by reference.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A pixel arrangement of an organic light emittingdisplay device with a plurality of repeatedly arranged sub-pixel groups,each of the sub-pixel groups comprising: two first sub-pixels in i-thand (i+2)-th lines (i is a natural number); two second sub-pixels in(i+1)-th and (i+3)-th lines; and two third sub-pixels in the (i+1)-thand (i+3)-th lines, wherein the first sub-pixel in the (i+2)-th line isdisposed between the second sub-pixel in the (i+1)-th line and thesecond sub-pixel in the (i+3)-th line, wherein the first sub-pixel inthe (i+2)-th line is disposed between the third sub-pixel in the(i+1)-th line and the third sub-pixel in the (i+3)-th line, wherein thei-th and (i+2)-th line include only the first sub-pixels, wherein thefirst sub-pixel is at least two times longer than the second sub-pixelalong a first direction, and wherein the second sub-pixels and the thirdsub-pixels are alternately arranged in a same line, the (i+1)-th and(i+3)-th line including only the first and second sub-pixels.
 2. Thepixel arrangement of the organic light emitting display device accordingto claim 1, wherein the second sub-pixel and the third sub-pixel arelonger than the first sub-pixel along a second direction.
 3. The pixelarrangement of the organic light emitting display device according toclaim 1, wherein the first sub-pixel is a blue sub-pixel, the secondsub-pixel is a red sub-pixel, and the third sub-pixel is a greensub-pixel.
 4. An organic light emitting display device including aplurality of repeatedly arranged sub-pixel groups, each of the sub-pixelgroups comprising: two first sub-pixels in i-th and (i+2)-th lines (i isa natural number); two second sub-pixels in (i+1)-th and (i+3)-th lines;and two third sub-pixels in the (i+1)-th and (i+3)-th lines, wherein thefirst sub-pixel in the (i+2)-th line is disposed between the secondsub-pixel in the (i+1)-th line and the second sub-pixel in the (i+3)-thline, wherein the first sub-pixel in the (i+2)-th line is disposedbetween the third sub-pixels in (i+1)-th line and the third sub-pixel inthe (i+3)-th line, wherein the i-th and (i+2)-th line include only thefirst sub-pixels, wherein the first sub-pixel is at least two timeslonger than the second sub-pixel along a first direction, and whereinthe second sub-pixels and the third sub-pixels are alternately arrangedin a same line, the (i+1)-th and (i+3)-th line including only the firstand second sub-pixels.
 5. The organic light emitting display deviceaccording to claim 4, wherein the second sub-pixel and the thirdsub-pixel are longer than the first sub-pixel along a second direction.6. The organic light emitting display device according to claim 4,wherein the first sub-pixel is a blue sub-pixel, the second sub-pixel isa red sub-pixel, and the third sub-pixel is a green sub-pixel.
 7. Anorganic light emitting display device including a plurality of lightemitting region groups, each of the light emitting region groupscomprising: two first light emitting regions in i-th and (i+2)-th lines(i is a natural number); two second light emitting regions in (i+1)-thand (i+3)-th lines; and two third light emitting regions in the (i+1)-thand (i+3)-th lines, wherein the first light emitting region in the(i+2)-th line is disposed between the second light emitting region inthe (i+1)-th line and the second light emitting region in the (i+3)-thline, wherein the first light emitting region in the (i+2)-th line isdisposed between the third light emitting region in the (i+1)-th lineand the third light emitting region in the (i+3)-th line, wherein thei-th and (i+2)-th line include only the first light emitting regions,wherein the first light emitting region is at least two times longerthan the second light emitting region along a first direction, andwherein the second light emitting region and the third light emittingregion are alternately arranged in a same line, the (i+1)-th and(i+3)-th line including only the first and second light emittingregions.
 8. The organic light emitting display device according to claim7, wherein the second light emitting region and the third light emittingregion are longer than the first light emitting region along a seconddirection.
 9. The organic light emitting display device according toclaim 7, wherein the first light emitting region is configured to emitblue light, the second light emitting region is configured to emit redlight, and the third light emitting region is configured to emit greenlight.
 10. An organic light emitting display device including aplurality of sub-pixel circuits and a plurality of light emitting regiongroups, each of the light emitting region groups comprising: two firstlight emitting regions in i-th and (i+2)-th lines (i is a naturalnumber); two second light emitting regions in (i+1)-th and (i+3)-thlines; and two third light emitting regions in the (i+1)-th and (i+3)-thlines, wherein the plurality of sub-pixel circuits comprises firstsub-pixel circuits connected to the first light emitting regions, secondsub-pixel circuits connected to the second light emitting regions, andthird sub-pixel circuits connected to the third light emitting regions,wherein the first light emitting region in the (i+2)-th line is disposedbetween the second light emitting region in the (i+1)-th line and thesecond light emitting region in the (i+3)-th line, wherein the firstlight emitting region in the (i+2)-th line is disposed between the thirdlight emitting region in the (i+1)-th line and the third light emittingregion in the (i+3)-th line, wherein the i-th and (i+2)-th line includeonly the first light emitting regions, wherein the first light emittingregion is at least two times longer than the second light emittingregion along a first direction, and wherein the second light emittingregion and the third light emitting region are alternately arranged in asame line, the (i+1)-th and (i+3)-th line including only the first andsecond light emitting regions.
 11. The organic light emitting displaydevice according to claim 10, wherein each of the first sub-pixelcircuits, the second sub-pixel circuits, and the third sub-pixelcircuits include: a first transistor; a second transistor; and acapacitor, wherein the capacitor is configured to store a potentialaccording to a video signal supplied from a video signal line via thesecond transistor, and wherein the first transistor is configured toflow a drive current according to the potential.
 12. The organic lightemitting display device according to claim 11, wherein the second lightemitting region and the third light emitting region are longer than thefirst light emitting region along the second direction.
 13. The organiclight emitting display device according to claim 12, wherein the firstlight emitting region is configured to emit blue light, the second lightemitting region is configured to emit red light, and the third lightemitting region is configured to emit green light.